Client:

Molycorp, Inc., Questa Division

Project Description:

The revised closure plan for the Questa tailings facility calls for the placement of a store-and-release cover at closure consisting of local alluvial material. An alternatives evaluation and initial soilcover modeling, completed in preparation for the closure plan, had indicated that this cover type would be the most suitable cover for such a large (260 ha) tailings facility, located in the semi-arid region of northern New Mexico. The revised closure plan also stipulated that infiltration test plots be installed to measure cover performance directly and to calibrate the soilcover model to be used for a final determination of alluvial cover thickness.

Robertson Geoconsultants Inc. was retained by Molycorp to design, construct and monitor the infiltration test plots. The final design of the test plots was based on extensive 1D and 2D soilcover modeling and discussions with local (state) regulators. The test plots were constructed in the summer of 2000 with the approval of the state agencies. The test plot study consists of two closed lysimeters covered with 0.23m (9") and 0.60m (24") of alluvial soil over backfilled tailings, respectively, and an instrumented deep in-situ tailings profile with a 0.28m (11") thick alluvial cover with existing grass/shrub vegetation. All test plots were instrumented with temperature/suction sensors and moisture content sensors to monitor moisture movement in the cover/tailings profile. The lysimeters are free-draining and outflow is collected and monitored continuously using a tipping bucket. In addition, a climate met station was erected to measure all climate parameters required for soilcover performance modeling.

Robertson Geoconsultants Inc. is currently assisting Molycorp with the on-going maintenance, data collection and interpretation of the test plot information. The monitoring data are made available to the State agencies in the form of Quarterly monitoring reports. RGC has recently calibrated a soilcover model using the 1st Year monitoring results from the test plots. This calibrated model has been used to predict the performance of the proposed store-and-release cover for various cover depths, vegetation covers and climate conditions. Additional model calibrations and further cover performance predictions are planned for the future.

Services Provided:

• Design of infiltration test plots
• Installation of infiltration test plots & climate station
• Monitoring & maintenance of infiltration test plots
• Verification & interpretation of test plot data
• Calibration of soil cover model using test plot data
• Prediction of cover performance (using SOILCOVER)

• Wels, C., Fortin, S., and Loudon, S.(2002) : Assessment of Store-And-Release Cover for Questa Tailings Facility, New Mexico. Paper presented at the "Tailings & Mine Waste Conference", January 27-30, 2002, Fort Colins, Colorado.
• Wels, C., O'Kane, M., and Fortin, S. (2001) : "Assessment of Water Storage cover for Questa Tailings Facility, New Mexico". Paper presented at the American Society for Surface Mining and Reclamation Conference, Proceedings Volume 2, pp.500-513, June 2001, Albuquerque, New Mexico.